The Fukushima facility was immediately shut down after the earthquake – but the nuclear fuel would still have been giving off a huge amount of heat.

The Fukushima facility was immediately shut down after the earthquake – but the nuclear fuel would still have been giving off a huge amount of heat.

Water provided by back-up diesel generators should cool it down, but it looks like they failed when the tsunami hit the power ­station, which is right on the coast.

The water in the reactor then started to boil.

There was a build-up of steam, which increased the pressure in the reactor, and hydrogen and radioactive caesium and iodine would also have been in that mix, because of fuel failure.

The pressure had to be dealt with to prevent the risk of meltdown or an explosion, so the operators vented steam and other gases into the outer ­container.

That move explains ­reports before yesterday’s explosion of slightly ­increased levels of radiation from the plant.

By releasing gases into the air in the ­containment building, the hydrogen and oxygen combined – an ­extremely volatile mix.

I believe this led to the explosion – and there were reports of an aftershock just before the blast, which could have produced the spark required, although that is just speculation.

Thankfully, although the explosion was spectacular, it wasn’t devastating and it seems the force was not sufficient to breach the reactor’s metal shell.

The relative lack of further radioactive emissions after the explosion would seem to support the official claims that the reactor suffered no further damage.

The authorities seem to have taken a decision early on that they don’t care about saving the plant as an ­operational facility – it is 40 years old and near the end of its life anyway. That’s why flooding the reactor with sea water should be a very effective way of cooling it down and preventing an immediate meltdown.

Now experts will have to take time to assess the level of lasting ­contamination so they can work out when to let people back into their homes.

They will also have to look at ways of disposing of the fuel, but that can wait until some time after the initial threat has been ruled out.

&#61592;MALCOLM Grimston is an ­Associate Fellow at the Royal Institute of ­International Affairs, Chatham House